Long-term service evaluation of a pultruded carbon/glass hybrid rod exposed to elevated temperature, hydraulic pressure and fatigue load coupling

In the present paper, a pultruded glass fiber shell (GFS) and carbon fiber core (CFC) hybrid rod (HFRP) with the diameter of 19 mm was exposed in an underground oil well. The exposure environment of elevated temperature (T), hydraulic pressure (P) and fatigue load (L) coupling was realized, including two exposure conditions, T = 25 degrees C, P = 0.5 MPa, L-max = 70 kN and T = 90 degrees C, P = 18 MPa, L-max = 50 kN. The long-term evaluation of mechanical, thermal properties and microstructure of HFRP rods was conducted to reveal the evolution mechanism during exposure. It can be observed that the short beam shear strength and interface shear strength of HFRP rods decreased obviously with the exposure time. Higher exposed temperature and hydraulic pressure aggravated the degradation through accelerating the diffusion of water molecule into HFRP rods. In contrast, higher fatigue stress level played an insignificant effect owing to the low difference of fatigue stress level. During exposure, the diffusion of water molecule caused the hydrolysis and plasticizing of resin and increased the pore volume content (similar to 3%) in the rod. Plenty of bound water (> 90%) formed and weaken the Van der Waals force and hydrogen bond between fiber and resin, finally leading to debonding of fiber/resin interface, especially for the higher exposed temperature and hydraulic pressure. The fatigue tests of HFRP rods immersed at 60 degrees C/20 MPa for one year in the laboratory were conducted to simulate the fatigue behavior exposed in the underground oil well. Furthermore, the residual fatigue life of HFRP rods was predicted to be more than 15.55 years. When the designed fatigue life of HFRP rods in actual exposure was 10 years, the maximum applied fatigue load was predicted to be 144.7 kN.